以定语从句为例来说明这个问题。

The girl who is wearing red coat is my sister.

在这个句子中 主语 the girl 受定语从句 who is wearing red coat 的修饰，谓语是 is，表语是 my sister。the girl 是先行词，who 就是引导词，也就是 关系代词。

再看一个例句:

The man over there who you were just talking with is my brother.